a)Filter是SUN公司提供的一个资源过滤器接口，不同的Web容器有着不同的实现
b)Filter位于Web服务器和Web资源（Servlet/Jsp/Html）之间
c)过滤请求和响应二者
d)Filter可以进行简单判段，是否将请求放行给Web资源
e)Filter的开发过程：

>类 implements javax.servlet.Filter接口
>在web.xml文件配置Filter过滤器，告之Web服务器有过滤器的存在
web.xml中的配置信息如下：
<filter>
<filter-name>FilterDemo1</filter-name>(过滤器，可以随意，但要和filter-mapping中的name一致)
<filter-class>cn.web.filter.FilterDemo1</filter-class>(过滤器全路径)
</filter>
<filter-mapping>
<filter-name>FilterDemo1</filter-name>(过滤器名，同上)
<url-pattern>/*</url-pattern>(过滤器能够过滤的资源路径，不是用户在URL中访问的路径)
</filter-mapping>

ShowServlet

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ShowServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("test/html; charset=UTF-8");
        response.getWriter().write("欢迎学习servlet下篇");

    }

}

FilterDemo1

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

//自定义Filter过渡器
public class FilterDemo1 implements Filter {
    public void destroy() {
    }
    public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {

        System.out.println("过滤器+++doFilter()");
        //放行请求[类似于函数调用]
        chain.doFilter(request,response);
        System.out.println("放行请求[类似于函数调用]");

    }
    public void init(FilterConfig filterConfig) throws ServletException {
    }
}

web.xml配置

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <filter>
        <filter-name>FilterDemo1</filter-name>
        <filter-class>cn.web.servlet.filter.FilterDemo1</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDemo1</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    </web-app>

原文地址：http://blog.51cto.com/357712148/2105072